Continuous measurement of blood PCO2 and PO2 is useful in monitoring patients suffering from severe respiratory failure. For example, recent work has shown that optimum "PEEP" therapy can be achieved by monitoring blood gases rather than compliances and pressures. We have developed a simple system consisting of an intravascular catheter probe which extracts a few nanoliters of CO2 and O2 from the blood and transports them to a sensitive gas analyzer. A simple calorimeter using the heat of reaction of CO2 with LiOH provides a sensitive and stable CO2 instrument. Oxygen is measured coulometrically using a fuel cell. These two techniques demonstrate superior performance over the conventional Severinghaus and Clark electrodes for long term measurements and are considerably less expensive than mass spectometry. The precision and accuracy of the system over 24 hour periods will be demonstrated.